There are many critical factors the pilot of an aircraft must consider with an aircraft before departure. One of those factors is the correct servicing levels of the internal fluids within the aircraft landing gear struts. Aircraft manufacturers determine, certify and publish maximum gross take-off and landing weight limitations of the aircraft. Aircraft manufacturers also have restrictions regarding the maximum allowable weight the aircraft landing gear and other supporting structures can safely absorb, when the aircraft lands. These landing weight restrictions are often determined not by how much weight the landing gear can safely handle at any single landing, but more as to the fatigue life of the landing gear system, in relation to the life expectancy of the aircraft, as a whole. The aircraft manufacturer must consider the possibility that bad weather at the airport or poor landing skills of less experienced pilots might cause “hard landing events”, which put much greater strain on the components and drastically shorten the anticipated fatigue life of the components. For instance, a heavily loaded aircraft making a smooth landing puts less strain on the aircraft and landing gear system, than a lightly loaded aircraft which lands either abruptly or asymmetrically, where one of the main landing gear makes ground contact first and must endure all of the force of the initial impact. Aircraft manufacturers which offer their airplanes through lease arrangements often find that after the initial lease period, it difficult to sell or re-lease the returned, mid-life aircraft, when the aircraft are returned with an expensive component such as the landing gear system, “run-out” to the absolute limits of its useful life.
Telescoping type landing gear have a sealed interior chamber that contains the internal fluids. The internal fluids include oil and nitrogen gas. Nitrogen gas is used for several reasons, one of which is to minimize corrosion of strut components; while another is that nitrogen gas is an inert gas, and, unlike oxygen gas, will not promote combustion.
In an airline operation, pilots will walk around the aircraft, at the departure gate, inspecting the aircraft. Part of that inspection process is to determine if the landing gear struts are properly serviced. As a “rule-of-thumb” pilots typically look for 4 inches of exposed chrome on the landing gear strut's telescopic piston, which supports the weight of the aircraft. If the pilot inspection finds a strut with only 2 inches of exposed chrome, the pilot will report the “low strut” to airline maintenance, and a technician will be sent out to correct the problem. Often, at smaller airports where the airline does not maintain full-time maintenance personnel, the airline will share the services of other airline technicians. Many of these airports sometimes have limited resources for maintaining the aircraft. If a landing gear strut is in need of additional nitrogen gas, and there is no bottle of compressed nitrogen gas available; landing gear servicing manual do allow for the use of compressed air, in place of the nitrogen gas. This is the point where oxygen can possibly be introduced to the inside of the landing gear, thus create an environment which will promote internal corrosion. If compressed air is introduced into the strut and the technician (possibly being an employee of a different airline) does not contact the maintenance department for the aircraft that was just serviced, the oxygen can remain within the landing gear for a long period of time. If this landing gear has a history of being identified as low, requiring a servicing event, chances are the same landing gear will require additional servicing, where additional oxygen can be introduced into the strut. Thus the environment which can promote corrosion can compound. These maintenance mis-practices are well known within the airline industry as well as by the aircraft manufacturers, but the manufacturers have no way to correct the problem for introduction of oxygen to the landing gear struts . . . thus the manufacturers will often assume a worst-case scenario, when they publish the Calendar Life limitations for the landing gear overhaul cycles.
An aircraft manufacturer must determine inspection and/or life cycle limitations based on estimated wear and tear on the landing gear systems by any given operator. There are two primary limitations which aircraft landing gear manufactures place upon their equipment, in an attempt to insure the landing gear strut remains in a robust configuration. These two limitations are “Number of Landing Cycles” and “Calendar Life” limitation. Number of Landing Cycles is based upon actual utilization of the aircraft in terms of landing events. As shown by examples below, some aircraft experience high utilization, while others experience low utilization. Calendar Life is based upon the manufacturer's concern regarding corrosion of internal landing gear components. These components are not subject to visual inspection unless the aircraft is removed from service and the strut disassembled. Typical aircraft operated by most of the major airlines, are manufactured to FAA Regulations—Part 25 “design criteria” (ie: Part 25 aircraft are for example Boeing 737, 747, 757, 767, 777, etc.) and have a Number of Landing Cycle limitations of around the 20,000 cycles, along with a Calendar Life limitation of about 120 months. Assuming a typical airline carrier aircraft has a daily utilization of 6 flights per day, further assuming 350 active flight-days per year; after a period of 111 months, that aircraft would fly and cycle the landing gear 19,980 times; being 20 cycles prior to the 20,000 Landing Cycle limitation. That same 111 months would be 9 months prior to the Calendar Life limitation.
For a better understanding as to the variety of aircraft utilization patterns, as an initial example: airlines such as Southwest Airlines, with their typical 45 minute flight-leg and 14 minute airport turn-times, have their aircraft departing every hour, on the hour. This allows the airline to get up to 14 flights per day for each aircraft. This high utilization has that aircraft reaching 19,740 landing cycles in just 47 months, 73 months before the Calendar Life limitation.
To use another example for an airline that has large international operations, such as American Airlines: changing the daily utilization assumption to that of a wide-body Boeing 777, used primarily to fly passengers from Dallas/Ft Worth to Paris, France in daily operations, that aircraft has only 2 landing cycles per day. During the 120 months of Calendar Life limitation, the Landing Cycle total would be only 7,200 landing events, against a 20,000 Landing Cycle life limitation.
The question then arises . . . how can one get an increase in the Calendar Life limitation? Internal corrosion, which degrades landing gear structural integrity, is primarily caused by the introduction of oxygen to the inside the landing gear strut. All landing gear strut maintenance procedures identify nitrogen gas as the preferred gas to be used to inflate a landing gear strut. Nitrogen is an inert gas, and does not promote corrosion within the landing gear strut.
The prior art which offer aids in monitoring landing gear health and the servicing levels of fluid and gas volumes within the landing gear strut are well known and well documented. Reference may be made to Technical Paper #02WAC-19, by Sidney G. Allison, NASA Langley Research Center—Ultrasonic Measurement of Aircraft Strut Hydraulic Fluid Level, which teaches the installation of sonic sensors to the external surface of the landing gear, with such ultra-sound patterns monitored to detect fluid and gas separation within the strut. US Patent Application US 2006/0144997 A1,—Gear, R. Kyle Schmidt, et al.—Method and System for Health Monitoring of Landing Gear—teaches the addition of various sensors to landing gear brakes, tires, hydraulics and electrical systems, and switches which initiate landing gear strut deployment and use. U.S. Pat. No. 4,092,947, Jean P. Labrecque—teaches the utilization of a sliding rod, traveling with the telescopic movement of the landing gear strut piston, where the sliding rod will rupture a strategically located disk, when oil level is low. US Patent Application US 2007/0069072 A1, William E. Luce—Aircraft Shock Strut Having a Fluid Monitor—teaches the installation of a fiber-optical, liquid sensing probe, inserted into the telescopic landing gear, to monitor oil levels. Prior art by this inventor (Nance) U.S. Pat. No. 7,274,309 and U.S. Pat. No. 7,274,310 which measure aircraft vertical velocity and thus the Kinetic Energy generated at initial touch-down. The prior art including the prior art of this inventor (Nance) U.S. Pat. No. 7,193,530 teaches landing gear life limit escalation through the monitoring of additional landing load data, accumulated with every aircraft landing event, to build an actual life history of the landing gear, to be used in comparison of the aircraft manufacturers' assumption of landing gear use or possible abuse, to develop the documentation necessary, with engineering review, to allow increases in the life limitation of the aircraft landing gear system.
In addition to causing corrosion, the introduction of oxygen into the inside of the landing gear can create an environment which will promote internal combustion. This internal combustion within the landing gear strut will be in the form of what is commonly known as a “diesel effect.” The diesel effect happens as the aircraft lands, compressing the strut. The weight of the aircraft, transfers an equivalent landing load as the landing gear come into contact with the runway. The dissipation of these landing loads generates a large amount of internal heat within the landing gear strut. The landing gear strut uses a method of squeezing hydraulic fluid, through a small internal orifice within the landing gear strut. The fluid friction of the hydraulic oil squeezing through the orifice generates heat. The hydraulic fluid will atomize, thus reduce to tiny particles or a fine spray, as it passes through the orifice; sometimes creating foam as it mixes with the compressed gas. The hydraulic fluid used in typical aircraft landing gear struts is an H-5606 mil-oil and said oil is quite flammable. The compression of the strut increases internal strut pressure and thus increases heat within the strut chamber, while the flammable fluid is distributed within the oxygen contamination of the strut; thus a diesel explosion can occur.